Fully automatic transmission systems, both for heavy duty vehicles such as heavy duty trucks, and for automobiles, that sense throttle openings or position, vehicle speed, engine speed and the like, and automatically shift the vehicle transmission in accordance therewith, are well known in the prior art. Such fully automatic gear change transmission systems include automated transmissions wherein pressurized fluid is utilized to frictionally engage one or more members to other members or to a ground to achieve a selected gear ratio as well as automated mechanical transmissions utilizing electronic and/or pneumatic logic and actuators to engage and disengage mechanical (i.e., positive) clutches to achieve a desired gear ratio. Semi-automatic or manual-automatic transmission systems utilizing electronic control units that sense throttle position, engine speed, input shaft speed, output shaft and/or vehicle speed, and utilize automatically controlled fuel throttle devices, gear shifting devices, and/or master clutch operating devices to substantially automatically implement operator selected transmission ratio changes are also known in the prior art.
While such semi-automatic mechanical transmission systems are very well received, as they are somewhat less expensive than a fully automatic transmission systems, allow manual clutch control for low speed operation, and/or do not require automatic selection of the operating gear ratio, they may be too expensive for certain applications as a relatively large number of sensors and automatically controllable actuators, such as a master clutch and/or a fuel throttle device actuator, are required to be provided, installed, and maintained. However, with modern electronic controls for heavy duty truck engines, the majority, if not all, of the sensors are already present for providing feedback relating to engine speed, vehicle speed, throttle position, etc. Thus, an implementation of a semi-automatic transmission or manual-automatic transmission can be achieved with minor modifications to the hardware, particularly those vehicles including an electronic engine controller. Such systems require relatively sophisticated algorithms to provide reliable operation particularly with respect to the upshift and downshift of the automatically controlled gear ratios of the manual-automatic transmission.
During an automatic shift attempt, the shift may be aborted, or otherwise not completed, due to a number of reasons such as a failed solenoid, failed sensors, failure to complete certain events within a specified time period, and the like, or by operator interaction such as by depressing the clutch. In such a case, it is desirable to engage, or attempt to engage, a gear so that the transmission is not left in a neutral state for an extended period of time. The control algorithm that forms a part of the present invention provides a highly reliable mechanism for commanding the safest and/or most likely to engage transmission gear after an aborted shift attempt.